An image display system is known that allows image data to be sent from a computer to an external output device such as a projector via a LAN or the like and thus allows an image to be displayed. For such an image display system, there is known a technique of transferring only a part of image data including a changed part of the image in order to reduce the volume of data transfer from the computer to the external output device, as disclosed in Japanese Patent Publication No. JP-A-2005-33763.
However, in order to detect a change in the image, transmission of image data from a VRAM to a RAM and comparison of images by a CPU are carried out in the computer. These operations may cause a heavy load on the computer and it may take a long time before transferring image data.